The day after Roswell has my vote. That read was the precursor that initiated my studies in all the extraterrestrial-related matters related to the US government, primarily in the 1960s and 1970s era. The sheer amount of evidence in every way (may it be photographs, testimonials, studies, disclosure of people involved,etc.) back then is quite overwhelming. Seems like it was for them as well - the indiscreet quantity of flying apparatus in the south to the point of being a local normality, unexplainable leaps of technology, people coming out and talking, etc.